As you have already found out, the php-adsense-account-library is only working with the old adsense interface. And I think once the new interface gets out of beta, the old interface will be deactivated. So it would be a good idea to find a working solution for the new interface.
But I found one thing that could work:
Once logged in, I can download the following csv file from the old AND THE NEW interface:
It is a csv file with the impressions, clicks, CTR, eCPM and income of the current month. Perhaps it would be a good idea for the widget to download this file and parse it to get that information?